Rhesus monkeys infected with various strains of SIV/Delta have been evaluated for the presence of arthritis. The overall incidence of arthritis in SIV-infected rhesus is 57%. To evaluate the possibility of mucosal immunity playing a role in the development of reactive joint disease, a study of the ileum and colon of SIV-infected animals at 3 weeks PI was initiated to evaluate the early events of the disease. The role of ileal and colonic mucosal immunity was evaluated concerning lymphocytes, IgA, IgG producing cells, total cellularity, TNF-alpha, nitrotyrosine producing cells, CD8+, CD4+, CD3+, T-cell alpha/beta, T-cell gamma/delta cell populations of the lamina propria. Infected animals were compared with trauma/death animals with grossly normal digestive tracts. The total cellularity of the lamina propria of all animals remained similar and the range of B-lymphocytes in the infected and control monkeys was similar. IgA positive cells were markedly reduced (50%) in the infected monkeys. IgG positive cells were moderatedly increased in the infected animals. Macrophage numbers in the lamina propria were similar in the two groups. TNF-alpha and nitrotyrosine staining cells were markedly increased in the infected animals, indicating a disease activation of the macrophage produced cytokines of the lamina propria of the ileum and colon. CD4+ and CD8+ populations varied in the infected animals, generally increasing in number in the infected monkeys. T-cell alpha and T-cell gamma were markedly increased in the infected animals. The mucosa of all infected animals was PCB positive for SIV. The data base presented offers a complete microscipic scheme for evaluation of mucosal immunity.